Please don't mix and match sticks of ram on a system regardless of whether they are from the same specification. if you must upgrade your ram, look at them in pairs. If you lost a shoe out of a pair, would you be able to buy one at the store or would you be needing to buy a pair? similarly, always look at tram kits when you want to upgrade the ram from say 8GB to 16GB.
You might also want to list the rams you currently own and the ram you're looking at.